17.1 Physical Defenses

Physical Barriers and Mechanical Defenses

Before any immune cell activates or any antibody is produced, your body relies on physical and mechanical defenses to block pathogens from ever gaining entry. These defenses form the first line of innate immunity, and they're remarkably effective: the vast majority of microbes you encounter never make it past this stage.

Physical Barriers Against Pathogens

Skin is the body's largest and most obvious physical barrier. Several features make it so effective:

• The stratum corneum (outermost layer of the epidermis) is made of dead, keratinized cells packed tightly together. Keratin is a tough, waterproof protein that resists penetration by most microorganisms. Areas of heavy use can develop calluses, which thicken this barrier even further.
• Sebaceous glands secrete sebum, an oily substance containing fatty acids and glycerides that have direct antimicrobial properties.
• The skin surface maintains a low pH (roughly 4 to 6), which inhibits the growth of many bacterial species. This acidic environment is sometimes called the acid mantle, formed by a combination of sebum, sweat, and lactic acid produced by resident skin bacteria.

Mucous membranes line every body cavity that opens to the exterior, including the nose, mouth, eyes, respiratory tract, and gastrointestinal and urogenital tracts. Unlike skin, mucous membranes are moist and thinner, so they rely on additional strategies:

• Goblet cells secrete mucus, a sticky layer rich in glycoproteins that physically traps microorganisms before they can reach epithelial surfaces.
• Mucus also contains antimicrobial enzymes like lysozyme, which breaks down peptidoglycan in bacterial cell walls, and secretory IgA (sIgA) antibodies, which bind pathogens and prevent them from attaching to epithelial cells.

Mechanical Defenses

Physical barriers keep pathogens out, but mechanical defenses actively remove them:

1. Ciliary escalator — Cilia lining the bronchi and trachea beat in coordinated waves, sweeping mucus (along with trapped particles and microbes) upward toward the throat, where it can be swallowed or expelled.
2. Peristalsis — Rhythmic muscular contractions in the esophagus and intestines push food and any associated microbes through the GI tract, limiting the time pathogens have to colonize.
3. Flushing actions — The flow of urine washes microbes out of the urinary tract, and tears continuously wash the surface of the eyes. Both fluids also contain lysozyme.
4. Coughing and sneezing — These reflexes forcefully expel air (and pathogens) from the respiratory tract when irritants or microbes are detected.

The Microbiome as a Defensive Mechanism

The normal microbiota (or microbiome) refers to the community of bacteria, fungi, and other microorganisms that live in and on your body. These organisms are most abundant in the gut (especially the colon), on the skin, and on mucous membrane surfaces.

The microbiome defends you through several mechanisms:

• Competitive exclusion — Resident microbes occupy attachment sites and consume available nutrients (like carbohydrates and iron), leaving fewer resources for invading pathogens. This is sometimes called microbial antagonism.
• Production of antimicrobial substances — Some commensals produce bacteriocins (proteins like colicins and nisin that kill closely related bacterial strains) or generate organic acids (lactic acid, acetic acid) and hydrogen peroxide, all of which create an environment hostile to pathogens.
• Immune system priming — Interactions between commensal bacteria and host cells (through pattern recognition receptors like Toll-like receptors) help the immune system mature and calibrate its responses. This is part of the basis for the hygiene hypothesis, which proposes that reduced microbial exposure in early life may increase susceptibility to allergies and autoimmune diseases.

Components of Skin and Mucous Membrane Defenses

Skin defense components in more detail:

• Corneocytes in the stratum corneum are embedded in a lipid matrix that limits transepidermal water loss and blocks microbial penetration.
• Langerhans cells are specialized dendritic cells residing in the epidermis. They detect foreign antigens and present them via MHC class II molecules to T cells, bridging the physical barrier with the adaptive immune response. While Langerhans cells are part of adaptive immunity, their presence in the skin means the barrier itself is immunologically active.

Mucous membrane defense components in more detail:

• Epithelial cells are connected by tight junctions (formed by proteins like claudins and occludins), which seal the gaps between cells and prevent pathogens from slipping through.
• Beneath the epithelium, the lamina propria contains immune cells including dendritic cells, macrophages, and lymphocytes that can mount a rapid response if the barrier is breached.
• Mucosal-associated lymphoid tissue (MALT) provides organized, localized immune surveillance throughout mucous membranes. Specific types include gut-associated lymphoid tissue (GALT), which contains structures like Peyer's patches and M cells, and bronchus-associated lymphoid tissue (BALT) in the respiratory tract.
• Secretory IgA is the dominant antibody in mucosal secretions. It exists as a dimer joined by a J chain and is transported across epithelial cells. sIgA neutralizes pathogens by blocking their ability to adhere to mucosal surfaces.

Additional Innate Defense Mechanisms

A few more components round out the physical defense picture:

• Antimicrobial peptides like defensins and cathelicidins are produced by epithelial cells and immune cells. They act as natural antibiotics by inserting into and disrupting bacterial cell membranes.
• Biofilms are structured communities of microorganisms encased in a self-produced matrix. In the context of normal microbiota, biofilms on mucosal surfaces can be protective by occupying space and resisting pathogen colonization. However, pathogenic biofilms (for example, on medical devices or in wounds) are a significant clinical problem because they resist both immune defenses and antibiotics.